1. Field of the Invention
The present invention is generally in the field of electrical circuits and systems. More specifically, the present invention is in the field of wireless communications circuits and systems.
2. Background Art
Transceivers are typically used in communications systems to support transmission and reception of communications signals through a common antenna, for example at radio frequency (RF) in a cellular telephone or other wireless communication device. A transmitter routinely implemented in such a transceiver may utilize several processing stages to condition and preamplify a transmit signal prior to passing the transmit signal to a power amplifier (PA). For example, the transmit signal may originate as a digital signal generated by a digital block of the transmitter. That digital signal is then typically converted into an analog baseband signal, by means of a digital-to-analog converter (DAC), for instance. The analog baseband signal may then be filtered using a low-pass filter (LPF) and up-converted to RF by a mixer. Subsequently, the up-converted signal can be processed by a PA driver, which then passes the preamplified transmit signal to the PA for additional amplification prior to transmission from the transceiver antenna.
As advances in electronics fabrication technologies enable the manufacture of ever smaller wireless transmitters, the need to implement area and power conserving circuit architectures becomes increasingly acute. As a result, passive mixer designs are becoming more desirable for use in up-converting baseband signals to RF due to their significantly reduced power and area requirements when compared with conventional active mixer designs, such as Gilbert cells, for example. In addition, passive mixer designs can provide excellent image rejection and very low local oscillator feedthrough across most gain ranges. Moreover, the devices used to implement passive mixers can be selected so as to scale with advances in semiconductor fabrication technology, thereby advantageously providing a persistent design solution.
Despite their potential advantages, the widespread use of passive mixers in wireless communications systems has been hindered by significant implementational challenges. For example, when compared to active mixers such as Gilbert cells, passive mixer designs typically require a considerably larger driving current, as well as requiring that the stage prior to the passive mixer provide a low output impedance. Moreover, the driving current and low prior stage output impedance requirements must be met without significantly impacting system performance, or substantially compromising the reductions in area and power resulting from passive mixer use.
Thus, there is a need to overcome the drawbacks and deficiencies in the art by providing a high performance buffer configured to provide high linearity, low output impedance, and low noise, and suitable for use as a pre-mixer buffer for a passive mixer implemented in a wireless communications system.